This invention relates generally to data communications. More particularly, this invention relates to a technique for line of sight laser communications.
Current line of sight communications use microwave technologies, and some communications work has been done with laser technology. Solid state fiber lasers have been developed for commercial communication applications. Picosecond pulses (ca 50 ps) are currently being used for long distance (transpacific) soliton communication. Much shorter pulses ( less than 100 Fs) have been generated with fiber sources, pumped by diode lasers. Such systems have the advantages that they are being tailored for communications, the bandwidth that is used is at an xe2x80x9ceye safexe2x80x9d wavelength, and fast modulation techniques have been or are being developed. There is still work to be done, but the fact that the light is confined to a narrow waveguide helps improve the speed of modulation. The disadvantages of such systems is that they have a lossy transition from fiber to air, they have a larger diffraction angle at longer wavelengths, the minimum size beam at 20 km is about 10 m, there is a low average and peak power (pJ/pulse), and a low repetition rate (1 to 10 MHz) exists.
Solid state Nd vanadate lasers (Nd:YVO_4) have also been used in prior art communication systems. These diode pumped lasers produce a train of pulses of about 5 ps duration (which could be compressed externally to the laser to 100 fs). The repetition rate is typically 100 MHz. The advantage of systems of this type are high efficiency, high average power-up to 100W in an infrared beam, 20W in the green, which can be frequency-tripled to a wavelength of 355 nm. The typical pulse energy in the green is 0.2 uJ/pulse. Pump diodes for this type of laser have a long lifetime; they are actively under development and they are relatively cost effective. In addition, these systems can provide diffraction limited beams in the green and UV. For the green, the minimum spot size at 20 km is about 2 m. For the UV, the minimum spot size at 20 km is about 1.5 m.
Solid state Cr:LiSAF Lasers (also Cr:LiSGAF and Cr:LiCAF) have been proposed for communication systems. Tunable pulse generation in the range of 820 nm to 880 nm has been demonstrated. Pulse duration as short as 20 fs has been obtained, but at very low average power. A maximum average power has been demonstrated at 1.1W, for continuous operation, while short pulse operation is only 0.5W. Advantageously, these systems have shorter pulses directly out of the laser, without compression. In addition, they have a shorter wavelength. The disadvantages of these systems is that they have less efficient pump diodes and less average power.
Ultra-high power laser pulses have also been proposed for data communications. It is possible to generate a stream of femtosecond pulses, each having a duration of less than 20 fs, and an energy of 10 mJ, at a repetition rate of up to 1 KHz. These sources are at 800 nm, can be frequency-tripled to 270 nm. The present costs are high. The average power is 10W. The information is stored in xe2x80x9cwordsxe2x80x9d coded in strings of pulses following the main filament forming pulse. The advantages of ultra-high power laser pulses include the fact that the laser pulses collapse in filaments of less than 100 um diameter, the filaments propagate through the atmosphere independently of atmospheric distortion or turbulence, and the spot size is  less than 100 um, so it is impossible to intercept without interrupting communication. In addition, there is back and forward scattered white light that is generated in the filament. This radiation can be detected both at the emitter and receiver. (It is this white xe2x80x9cconical radiationxe2x80x9d that has been detected at the source, for a propagation distance of 11 km). The disadvantage of these systems are their high costs, research is needed to establish its utility, and, if successful, the military may classify the technology. How long the channel will live is unknown. Channel life determines the length of the word that can follow the main pulse. How long the filament can be stable is also unknown.
The invention is a line of sight laser communication system with a laser to generate a laser signal with femtosecond pulses. A first grating spectrally disperses the femtosecond pulses of the laser signal. A programmable mask converts the femtosecond pulses of the laser signal into coded words. A second grating spectrally recombines the coded words of the laser signal. A telescope then launches the laser signal. The launched laser signal is received at a receiving telescope. A second laser generates a set of reference pulses. A non-linear crystal combines the set of reference pulses and the laser signal to create an output signal only when the laser signal and the reference pulses temporally coincide. A camera records the output signal.